Blue carbon is simply the term for carbon captured by the world's ocean and coastal ecosystems. Seagrass, mangroves and salt marshes along our coast "capture and hold" carbon, acting as something called a carbon sink. These coastal systems, though much smaller in size than the planet's forests, sequester this carbon at a much faster rate, and can continue to do so for millions of years. Most of the carbon taken up by these ecosystems is stored below ground where we can't see it, but it is still there. The carbon found in coastal soil is often thousands of years old!